14.5.4 Characteristics of BDB Tables

Each BDB table is stored on disk in two files.
The files have names that begin with the table name and have an
extension to indicate the file type. An .frm
file stores the table format, and a .db file
contains the table data and indexes.

To specify explicitly that you want a BDB
table, indicate that with an ENGINE table
option:

CREATE TABLE t (i INT) ENGINE = BDB;

The older term TYPE is supported as a synonym
for ENGINE for backward compatibility, but
ENGINE is the preferred term and
TYPE is deprecated.

BerkeleyDB is a synonym for
BDB in the ENGINE table
option.

The BDB storage engine provides transactional
tables. The way you use these tables depends on the autocommit
mode:

If you are running with autocommit enabled (which is the
default), changes to BDB tables are
committed immediately and cannot be rolled back.

If you are running with autocommit disabled, changes do not
become permanent until you execute a
COMMIT statement. Instead of
committing, you can execute
ROLLBACK to
forget the changes.

You can start a transaction with the
START
TRANSACTION or
BEGIN
statement to suspend autocommit, or with SET
autocommit = 0 to disable autocommit explicitly.

BDB tables can have up to 31 indexes per
table, 16 columns per index, and a maximum key size of 1024
bytes.

MySQL requires a primary key in each BDB
table so that each row can be uniquely identified. If you
don't create one explicitly by declaring a PRIMARY
KEY, MySQL creates and maintains a hidden primary
key for you. The hidden key has a length of five bytes and is
incremented for each insert attempt. This key does not appear
in the output of SHOW CREATE
TABLE or DESCRIBE.

The primary key is faster than any other index, because it is
stored together with the row data. The other indexes are
stored as the key data plus the primary key, so it is
important to keep the primary key as short as possible to save
disk space and get better speed.

This behavior is similar to that of InnoDB,
where shorter primary keys save space not only in the primary
index but in secondary indexes as well.

If all columns that you access in a BDB
table are part of the same index or part of the primary key,
MySQL can execute the query without having to access the
actual row. In a MyISAM table, this can be
done only if the columns are part of the same index.

Sequential scanning is slower for BDB
tables than for MyISAM tables because the
data in BDB tables is stored in B-trees and
not in a separate data file.

Key values are not prefix- or suffix-compressed like key
values in MyISAM tables. In other words,
key information takes a little more space in
BDB tables compared to
MyISAM tables.

There are often holes in the BDB table to
permit you to insert new rows in the middle of the index tree.
This makes BDB tables somewhat larger than
MyISAM tables.

SELECT COUNT(*) FROM
tbl_name is slow for
BDB tables, because no row count is
maintained in the table.

The optimizer needs to know the approximate number of rows in
the table. MySQL solves this by counting inserts and
maintaining this in a separate segment in each
BDB table. If you don't issue a lot of
DELETE or
ROLLBACK
statements, this number should be accurate enough for the
MySQL optimizer. However, MySQL stores the number only on
close, so it may be incorrect if the server terminates
unexpectedly. It should not be fatal even if this number is
not 100% correct. You can update the row count by using
ANALYZE TABLE or
OPTIMIZE TABLE. See
Section 13.7.2.1, “ANALYZE TABLE Syntax”, and
Section 13.7.2.5, “OPTIMIZE TABLE Syntax”.

Internal locking in BDB tables is done at
the page level.

LOCK TABLES works on
BDB tables as with other tables. If you do
not use LOCK TABLES, MySQL
issues an internal multiple-write lock on the table (a lock
that does not block other writers) to ensure that the table is
properly locked if another thread issues a table lock.

To support transaction rollback, the BDB
storage engine maintains log files. For maximum performance,
you can use the --bdb-logdir
option to place the BDB logs on a different
disk than the one where your databases are located.

MySQL performs a checkpoint each time a new
BDB log file is started, and removes any
BDB log files that are not needed for
current transactions. You can also use
FLUSH LOGS at
any time to checkpoint the Berkeley DB tables.

If you delete old log files that are still in use,
BDB is not able to do recovery at all and
you may lose data if something goes wrong.

Applications must always be prepared to handle cases where any
change of a BDB table may cause an
automatic rollback and any read may fail with a deadlock
error.

If you get a full disk with a BDB table,
you get an error (probably error 28) and the transaction
should roll back. This contrasts with
MyISAM tables, for which
mysqld waits for sufficient free disk space
before continuing.